Allstate Insurance Company said Friday it filed a $29.9 million lawsuit against 52 people in New York alleging they defrauded the insurer in a scheme that involved personal injury attorneys, doctors and other medical professionals.

The lawsuit is a civil case brought against people who are already the subject of a criminal case in a city known for insurance fraud at a time when the number of questionable insurance claims is rising nationally.

The lawsuit comes on the heels of a crackdown announced last week by New York insurance regulators on so-called "medical mills" in which doctors and clinics inflate bills or charge for services that were never provided to victims of a car crash. Auto insurers are required to pay for policyholders' medical bills regardless of who is at fault in a crash in states that have "no fault" insurance laws, such as New York and Connecticut.

All of the defendants in the Allstate civil case are under federal indictment and have been charged with health care fraud, racketeering and other charges related to allegedly scamming private insurers of more than $279 million by taking advantage of New York's no-fault laws.

The alleged scheme is believed to be the largest single no-fault auto insurance fraud ever. Since 2003, Allstate has filed 41 civil lawsuits in New York seeking a total of more than $227 million in damages related to fraud.

"In essence, honest, hardworking New Yorkers are paying a 'fraud tax'," said Krista Conte, an Allstate spokeswoman. "We need lawmakers to enact meaningful insurance reform that puts the citizens of New York first."

New York and Florida have particularly high rates of fraud related to people taking advantage of "no fault laws" on auto insurance, according to the Insurance Information Institute. States that have no-fault laws allow an auto policyholder to collect directly from his or her insurance company for medical and hospital expenses regardless of who was at fault in an accident.

In metropolitan New York City, about one in every five no-fault insurance claims appeared to have an element of fraud, and as many as one in three had inflated medical billing, according to the Insurance Information Institute and a study by the Insurance Research Council in January 2011.

"Over the period 2007 to 2010, the percentage of no-fault claims that were fraudulent or were inflated by excessive billing by unscrupulous medical care providers or by unnecessary medical services rose from 29 percent to 35 percent," Insurance Information Institute spokeswoman Loretta Worters said of New York City. "In the fall of 2010 alone, fraud was found in 22 percent of all New York City metropolitan area no-fault auto insurance claims and buildup, (or inflated costs,) in another 14 percent. By comparison, outside the city fraud was found in only 4 percent of no-fault claims settled and build-up in another 4 percent."

It's difficult to quantify the price tag of fraud, but there has been a rise nationally in the number of questionable claims.

The number of "questionable" insurance claims, meaning those identified by property-casualty insurers as potentially fraudulent, has increased steadily in recent years from 84,407 three years ago, to 91,797 two years ago and 100,450 last year, according to an analysis by the National Insurance Crime Bureau.

It's almost impossible to know how much of the increase in "questionable claims" is due to greater scrutiny by insurers and how much is due to consumers trying to scam insurers.

Insurance companies refer questionable claims to the National Insurance Crime Bureau, which is funded by the industry. One questionable claim can account for as many as seven different referrals.

In casualty insurance — not property — fake or exaggerated injuries account for 33 percent of referrals, followed by excessive medical treatment (16 percent), medical billing for treatment that wasn't actually rendered (10 percent), and injuries a person had prior to the accident (9 percent).

Total fraud referrals for questionable casualty claims rose 19 percent between 2009 and 2010, and rose again 16 percent between 2010 and 2011.

On May 1, the New York insurance regulators announced new regulations on the state's no fault laws to lessen fraud.

"These reforms will ensure that New Yorkers get the proper and timely treatment for legitimate injuries that they deserve, while closing loopholes that allow criminal medical mills to scam the system and drive up insurance premiums," said Benjamin M. Lawsky, New York Superintendent of Financial Services, who oversees insurance regulation in the state.